1. Field of the Invention
The present invention relates to a second harmonic generator, and more particularly to an apparatus for converting a fundamental wavelength light propagating in an optical wave guide to a second harmonic which efficiently propagates in the optical wave guide.
2. Related Background Art
A second harmonic generation (SHG) element which utilizes a non-linear optical effect has recently been developed for an optical wave guide. The SHG element converts a wavelength of a laser beam to one-half, and it converts an infrared ray to a visible ray or a visible ray to an ultraviolet ray. Accordingly, it has a great industrial value. In a band of 0.3-0.5 .mu.m which is difficult to oscillate by a semiconductor laser alone, a 0.4 .mu.m band coherent light is generated by a combination of a 0.8 .mu.m band semiconductor laser and an SHG element so that an integration density of an optical memory and a precision of a scanner are enhanced.
In a known SHG element, a wave guide for confining a light is formed by a proton exchange method on a LiNbO.sub.3 substrate having a large electro-optical effect
FIG. 1 shows a schematic view of a conventional SHG element. Numeral 1 denotes an optical wave guide made of a non-linear crystal such as LiNbO.sub.3, and numeral 2 denotes a high refractive index layer formed by a proton exchange method. Both an .omega.-light (fundamental wavelength light) and a 2.omega.-light (second harmonic light) are confined in this area. Numeral 3 denotes a prism coupler for coupling the .omega.-light to the wave guide 1, numeral 4 denotes a prism coupler for extracting the .omega.-light and the 2.omega.-light generated in the wave guide, numeral 5 denotes the .omega.-light directed to the prism coupler 3, and numerals 6 and 7 denote .omega.-light and 2.omega.-light emitted from the prism coupler 4, respectively.
In the SHG element, in order to efficiently convert the fundamental wavelength light (angular frequency .omega.) to the second harmonic light (angular frequency 2.omega.), it is necessary to set the phase velocities of the .omega.-wave and the 2.omega.-wave equal by phase matching. In the prior art optical wave guide, a mode diversity characteristic of the optical wave guide is utilized and the film thickness of the optical wave guide is controlled to make effective refractive indices for the .omega.-wave and 2.omega.-wave equal, or a temperature of a crystal is controlled to attain the phase matching.
However, the temperature control is very severe. As shown in FIGS. 2A and 2B, when the temperature is controlled with a target temperature tm (phase matching temperature), the second harmonic may not be generated for a long time period as shown by a period A-B because the second harmonic is generated only at the temperature tm. Accordingly, it has not yet been put into practical use.